Dental caries and hypersensitivity are two very common dental diseases. It is known that dental varnishes such as fluorides are capable to treat both dental caries and hypersensitivity. Fluorides form the protective layer of CaF2 on the tooth. As a result, dissolving of the tooth enamel, which chiefly consists of hydroxyapatite, under attack by acid is slowed down by forming acid resistant fluoroapatite. Fluorides also speed up the crystallization of hydroxyapatite and prevent further demineralization.
Commonly used fluoride sources are amino fluorides, stannous fluoride, acidulated phosphate fluoride, sodium monofluorophosphate and sodium fluoride. All these have certain advantages and limitations, for example amino fluorides such as N′,N′-tri-(polyoxyethylene)-N-hexadecylpropylene diamine dihydrofluoride; 9-octadecylamine hydrofluoride, hexadecylamine hydrofluoride and bis-(hydroxyethyl)-aminopropyl-Nhydroxyethyloctadecylamine dihydrofluoride, because of their cation activity, have a characteristic taste, which is difficult to mask. However, even low concentrations (0.001%-F) are highly effective. Stannous fluorides, on the other hand, are not stable in aqueous preparations, have a unpleasant metallic astringent taste, irritant to gingival and also cause the discoloration due to deposition of Sn3F3PO4. Therefore, products for prophylaxis with stannous fluoride had disappeared completely from the market for a long time.
Sodium fluoride (NaF) is a commonly used fluoride salt in varnish systems. Sodium fluorides have a basic pH, chemically stable when stored in plastic or polythene containers. NaF dissolves well in water and quickly releases very high fluoride concentrations. Sodium fluoride delivers a highly reactive fluoride ion; therefore, formulating it with a compatible abrasive is critically important for achieving the anti-caries benefit. These materials are not irritant to the gingival, and do not cause discoloration to teeth.
Acidulated phosphate fluoride (APF) on other hand is composed of NaF to which acid is added. The concentration of fluoride is 1.23%, the acid is in form of orthophosphoric acid the pH is 3.2. It is chemically stable when stored in plastic containers, and does not cause discoloration to teeth. The success of any topical fluoridated agent depends on its capability of depositing fluoride ions in the enamel as fluoroapatite and not only calcium fluoride. Compare to NaF, APF efficiently stimulates the fluoroapatite deposition on enamel by higher fluoride ion concentration and lowering the pH. Increase in the concentration of fluoride ions lead to formation of calcium fluoride and phosphate, while the presence of acid leads to break down of the outer enamel surfaces and release of calcium and phosphate. In both reactions phosphate formed. The increase in phosphate concentration causes the shift in the equilibrium of the reaction to right side that is in the direction of formation of fluoroapatite as well as hydroxyapatite crystals.
Sodium monofluorphosphates (NaMFP), unlike other ionic fluorides, consist of covalently bound fluoridated compound; thus it cannot go into solution by dissociation. It must first be hydrolyzed by bacterial phosphatases, which are present in the saliva and plaque. Therefore, NaMFP releases markedly less fluoride at very slower rate compare to other ionically soluble fluoride salts like NaF, SnF2, amino fluorides. Mechanistically, ionically bound fluoride acts through dissociation of the fluoride ion and the subsequent precipitation of calcium fluoride, the monofluorphosphates ion is probably incorporated into enamel and dentine as a complex ion.
Fluoride-containing products like toothpaste, fluoride tray, mouth rinse, tablets, are usually developed for topical applications with various concentrations and application forms. Fluoride uptake into enamel is a relatively slow process and takes a longer period of contact, thus fluoride varnishes have been developed which is typically a polymeric coating containing fluoride source that presents the fluoride in close proximity to enamel for prolonged time. Varnish method has additional advantages over the other treatment methods such as prolonged contact of fluoride agent with enamel surface, act as a slow release reservoir; the amount of fluoride permanently retained in the enamel is increased; prevent/or delay caries progression by the formation of acid resistant fluoroapatite/or flurohydroxyapatite; minimize the exposure to large quantities of fluoride, these could be applied easily; quickly without the need of professional prophylaxis; eat and drink following applications. Commonly used ingredients in fluoride varnish are sodium fluoride, rosin/and synthetic resin, various solvents, flavor additives, sweetener and pigments.
Although current varnishes have shown significant success in preventing dental caries and hypersensitivity, these systems have certain limitations. Conventional fluoride varnishes are made with tree rosins and synthetic resins that are partially dissolved by organic solvents. These rosin coatings are hydrophobic and do not release sufficient fluoride in an effective manner. Moreover, solvents such as hexane or heptanes are not very biocompatible. Other conventional fluoride varnishes contain polymers dissolved in solvent such as ethyl or butyl acetate. Ethyl and butyl acetate are harsh on oral tissue, and are barely tolerable by the patient. Additionally, many conventional fluoride varnishes leave a long lasting hard coat on the teeth that must be broken and picked from the teeth. Moreover, many conventional fluoride varnishes may have a yellow color and are not aesthetically pleasing to the patient. Existing varnishes that contain sodium fluoride contain little or no water (<5%). However, sodium fluoride is soluble in water; therefore, the fluoride is insoluble and precipitate in the hydrophobic solution creating a non-homogenous mixture resulting in formation of cluster of fluoride particles and in an irregular release of fluoride. In order to interact with tooth enamel, it is essential that the precipitated fluoride must first dissolve by aqueous saliva, which usually dissolves the fluoride available on the surface of film, and has difficulty in releasing the entrapped fluoride inside the film and that delay the release of fluoride. It is a further problem that the fluoride ions released can diffuse through the hydrophobic varnish film in the direction of the tooth surface only with difficulty, and therefore mostly migrate into the oral cavity. This means that although known products can release large amounts of fluoride, only a little fluoride reaches the enamel, thus high sodium fluoride concentrations necessary for adequate fluoridation increase the risk of fluorosis.